1 Field of the Invention
This invention is concerned with a fluidized catalytic cracking process wherein coke deactivated catalyst is subjected to stripping prior to coke burnoff.
2 Description of the Prior Art
The field of catalytic cracking has undergone progressive development since 1940. The trend of development of the FCC process has been to all riser cracking, use of zeolite-containing catalyst, heat balanced operation, and complete afterburning of CO to CO.sub.2 within the regenerator.
Other major trends in FCC processing have been modifications of the process to permit it to accommodate a wider range of feedstocks, in particular, stocks that contained more metals and sulfur than had previously been permitted in the feed to an FCC unit.
Along with the development of process modifications, and catalysts, which could accommodate these heavier, dirtier feeds, there has been growing concern about the amount of sulfur contained in the feed that ended up as SO.sub.x in the regenerator flue gas. Higher sulfur levels in the feed, combined with a more complete regeneration of the catalyst in the FCC regenerator, tended to increase the amount of SO.sub.x contained in the regenerator flue gas. Some attempts have been made to minimize the amount of SO.sub.x discharged to the atmosphere through the flue gas by providing agents to react with the SO.sub.x in the flue gas. These agents pass along with the regenerated catalyst back to the FCC reactor, and then the reducing atmosphere there releases the sulfur compounds as H.sub.2 S. Suitable agents for this purpose have been described in U.S. Pat. Nos. 4,071,436 and 3,834,031. Use of a cerium oxide agent is shown in U.S. Pat. No. 4,001,375.
Unfortunately, the conditions in most FCC regenerators are not the best SO.sub.x adsorption. The very high temperatures encountered in modern CO afterburning FCC regenerators tend to discourage SO.sub.x adsorption.
An example of a CO afterburning mode of operation using a conventional cracking catalyst to which has been added a metallic reactant which reacts with sulfur oxides is shown in U.S. Pat. No. 4,238,317, the entire contents of which are incorporated herein by reference. In this reference, metallic reactant reacts with SO.sub.x in the FCC regenerator to form stable sulfur compounds. This metallic reactant plus sulfur complex is circulated, along with the FCC catalyst, back to the FCC reactor where conventional cracking takes place. In the reducing atmosphere of the FCC reactor the metal and sulfur complex reacts to form a sulfide of the metallic reactant. The conventional FCC catalyst is withdrawn from the FCC reactor, along with the sulfide of the metallic agent and subjected to steam stripping at temperatures of 850 to 1200 F. The sulfide of the metallic agent reacts with water to form hydrogen sulfide gas and restore the metallic agent to a form in which it can react with additional sulfur oxides in the FCC regenerator.
A somewhat different approach to reducing SO.sub.x emissions from an FCC regenerator is described in U.S. Pat. No. 4,274,942, the entire contents of which is incorporated herein by reference. In this patent, spent catalyst is subjected to a partial oxidation treatment before entering the conventional CO afterburning FCC regenerator. The partial oxidation treatment, preferably conducted in the presence of steam, produces CO, CO.sub.2 and some H.sub.2 S. Catalyst circulates from the riser reactor to a conventional stripping zone within the riser reactor to the partial oxidation zone to the conventional CO afterburning regenerator. Because some of the sulfur compounds are released, as H.sub.2 S, during the partial oxidation, the H.sub.2 S produced never enters the regenerator and never appears as SO.sub.x in the regenerator flue gas. The environment within the partial oxidation, or sulfur rejection system, includes temperatures of 900 to 1300 F with injection of enough oxygen to combust about one third of the coke present.
A slightly different approach to removal of SO.sub.x from regenerator flue gas is shown in U.S. Pat. No. 4,284,494, the entire contents of which are incorporated herein by reference. The patentee recognized that the very hot conditions existing in a CO afterburning regenerator were not conducive to maximum absorption of sulfur oxides. In this patent, the hot flue gas was cooled, and this cooled flue gas contacted with regenerated catalyst. The regenerated catalyst adsorbs more SO.sub.x at the lower temperature than it does in the conditions existing within the FCC regenerator. Operation with an agent capable of reacting with or sorbing SO.sub.x is preferred.
Another way of minimizing SO.sub.x content in regenerator flue gas is to minimize the amount of spent catalyst, with its accompanying sulfur compounds, that enters the regenerator. Such an approach is disclosed in U.S. Pat. No. 3,926,778, the entire contents of which are incorporated herein by reference. The patentee discovered that high temperature soaking of spent catalyst alone or in combination with freshly regenerated catalyst restored enough activity to the catalyst so that it could be used for further catalytic cracking without regeneration. Catalyst flowed from the regenerator to a first riser reactor to a heat soak zone to a second riser reactor to a conventional stripping zone, and from there back to the catalyst regenerator to complete the cycle.
None of these approaches provided a completely satisfactory solution to the problem of minimizing SO.sub.x emissions from FCC regenerator flue gas. In studying the work that others had done, we discovered a way to significantly reduce SO.sub.x emissions from the flue gas, without requiring the addition of agents to adsorb SO.sub.x and without cooling the regenerator flue gas prior to contact with regnenerated catalyst. We also discovered a way to increase slightly the yield of valuable liquid products from the FCC process.